Telephone booths are often located in remote areas such as beaches, parks, side roads, rest areas and the like where it is difficult or uneconomical to run electrical power lines to supply dark or night time lighting for the telephone facility. Electrical code requirements in some localities and the cost of electrical power lines, as well as possibly requiring the use of transformers and the like for stepping down the voltage for lighting purposes at the telephone facility, can be quite expensive so there has been a problem in the past in economically supplying electrical energy for lighting purposes at the telephone facility. This lighting problem has been aggravated with the recent massive increase in electric energy costs. While systems using PV arrays or cells for charging storage batteries which are, in turn, used for night time lighting or other purposes, and which may or may not be controlled by light level sensors, have been well known, these have not been applied in the past to telephone booth usage.
The object of the present invention is to eliminate the requirement for electric power lines to supply electrical energy for operating the lamps or electrical equipment for outdoor telephone booths, and, more particularly, to economically incorporate PV arrays, storage batteries, and battery protective charging and control systems to telephone booths. According to the invention, a PV array, which may be mounted in the roof panel of a telephone booth or remote housing unit, or in some preferred situations, on a pole or aerial mast with the PV array being disguised as a sign or including sign indicia as part of the surfaces of the structure. The PV array in the preferred embodiment is constituted by one or more commercially available PV arrays connected electrically in parallel and supplying electrical energy to an electrical storage battery through a battery charge control unit which, in the preferred embodiment is a temperature compensated battery charge control regulator having a thermistor assembly attached thereto for providing a temperature compensated maximum charging voltage limit which matches the manufacturers' suggested limits for battery charging of the 12 volt storage battery to preclude overcharging and possible damage to the battery. If desired, various arrangements of the PV arrays can be used to supply higher or lower voltages. In the preferred embodiment standard commercially available PV arrays, such as the ARCO AS116-2300, are used to charge one or more storage batteries. Moreover, the battery in the preferred embodiment is a conventional 12 volt deep discharge battery which can withstand many discharging and recharging sequences or cycles and is preferrably housed in or on the base of the booth.
The circuitry uses of the PV array as an ambient light intensity sensor so as to assure that during daylight hours there is no electrical load connected to the battery and the electrical energy from the PV array is confined to charging the battery.
The choice of storage battery and PV array requires that consideration be given to effects of average daily solar insolation, prolonged periods of low solar insolation, low ambient temperature, coloumbic charging efficiency of the storage battery, and aging effects as well as the total electrical load. In those instances where insufficient solar insolation occasionally occurs over a prolonged period, the battery discharge limit control circuit provides a means to disconnect the load if the storage battery terminal voltage falls below a predetermined safe level. Exceeding the minimum safe discharge level on typical liquid electrolyte batteries shortens battery life and makes the battery electrolyte material susceptible to freezing. User sensing means controls light intensity level by pulse width modulation to conserve energy and extend operation during extended periods of low solar energy and/or darkness.
As noted above, the solar cells themselves are conventional but they may be mounted upon an aerial mast or pole so as to take them out of the reach of vandals and, also, in roadside type installations, locate them above the high dust or dirt level so that they will require less cleaning and maintenance while avoiding deterioration in the performance of the solar cells because of such dirt and dust. In this embodiment, the solar cell structure itself is preferably disguised as a sign so as to direct users to the phone facility from a greater distance. A due South exposure and at an angle suited to the latitude of the installation site is desirable for orientation relative to the path of the sun and for permitting water and dirt to run off. The sign can be a reflective sign and/or even be illuminated by a further fluorescent lamp for directing phone users to that phone facility.
It is contemplated that in a further embodiment the telephone booth will retain the normal fluorescent lamp (typically a circular lamp) and ballast or current limiting means which is powered by an electronic inverter or vibrator connected to the battery for converting and stepping up the 12 volt voltage to 110 volts AC for normal operation of the fluorescent lamp. Of course, the ambient light detector circuitry referred to earlier herein controls power to the inverter which, in turn, powers the fluorescent lamp.